This invention relates to electrical data pulse slicing, and more particularly to a method of and an arrangement for electrical data pulse slicing.
The invention has a particular but non-exclusive application in television receiver apparatus in or for use in a television system of a character in which coded data pulses, pertaining to alpha-numeric text or other message information, are transmitted in a video signal in at least one television line in field-blanking intervals where no picture signals pertaining to normal picture information are present.
A television system of the above character is described in United Kingdom patent specification No. 1,370,535. A conventional television receiver for the system includes, or has associated with it, additional means comprising a data acquisition circuit to extract the coded data pulses from a received video signal. The extracted coded data pulses are stored in a storage device of the additional means and, after a plurality of frame periods, an entity of related message information, for example a page of text, has been received and stored. The additional means also includes a decoding circuit for converting the stored message information into a video signal which is used to cause the display of the message information at the television receiver.
In the implementation of a practical data acquisition circuit for extracting coded data pulses from a received video signal, a difficulty that arises is to determine in the data acquisition circuit, a suitable bias voltage level, relative to the received signal level, which serves as a so-called "slicing level". Only pulse amplitudes in the received video signal which are greater than the slicing level are extracted as data pulses.
It is known to provide in the data acquisition circuit a data slicing arrangement which clamps the received video signal and slices it at a fixed d.c. level. However, this data slicing technique imposes the limitation that the level of the received video signal has to remain fairly accurately related to the fixed d.c. level at all times, which can cause problems, in particular when video signals may be received from different transmission sources.
With a view to avoiding the above limitation, adaptive data slicing techniques have been proposed. In a first known adaptive data slicing arrangement the slicing level is set automatically mid-way between positive and negative peaks of the received video signal. In a second known adaptive data slicing arrangement, for use where a block of coded data pulses is preceded by a sequence of clock run-in pulses, the slicing level for the block of coded data pulses is set to be the mean amplitude of the pulses that form the clock run-in sequence.
These two known adaptive data slicing arrangements mentioned above have, in particular, been developed for a data acquisition circuit for receiving Teletext data as broadcast in the United Kingdom by the BBC and the IBA using their respective CEEFAX and ORACLE data transmission systems which operate within the broadcast standards for the 625-line domestic television system as employed in the United Kingdom.
In order to slice Teletext data correctly off a received video signal, variations in amplitude of the Teletext, data due to its reception from different transmission sources, is not the only problem which has to be overcome. Further problems may be caused by: a weak signal in the presence of random noise; amplitude and group delay distortions which produce "overshoots" in the pulse waveforms in the video signal; and co-channel interference which produces low frequency variations in the video signal.
In an embodiment of the first known adaptive data slicing arrangement referred to above, two peak detectors are used to store the positive and negative peaks, respectively, of received Teletext data. Two storage capacitors are used for this purpose and the slicing level is set at the mean (50%) of the two capacitor voltages. In order that this arrangement is not unacceptably affected by co-channel interference, it requires a short "attack" (charging) time constant, whereas it requires a relatively longer "attack" time constant in order not to be unacceptably affected by "overshoots". Therefore a compromise has to be made when selecting the "attack" time constant, but a shorter time constant may be preferred so that co-channel interference can be followed when determining the slicing level. A shorter time constant also improves the performance of the arrangement in the presence of noise.
In an embodiment of the second known adaptive data slicing arrangement referred to above, an integrating capacitor is used to store the mean magnitude of the pulses that form the clock run-in sequence, and the slicing level is held at this mean magnitude for the duration of the block of coded data pulses that follow the clock run-in sequence. With this second arrangement "overshoots" have no significant effect on the slicing level, but the arrangement cannot follow (and thus cope with) co-channel interference. This arrangement may perform better than the first arrangement in the presence of noise. However, it is more complex in implementation in that it requires a timing circuit to gate out the clock run-in sequence on each occurrence thereof.